Method and a device for removing nitrogen oxides and sulphur trioxide from a process gas

ABSTRACT

A catalytic reactor ( 16 ) is provided for purposes of effecting therewith the removal of nitrogen oxides from a process gas (F) that includes at least two catalyst bed segments ( 48, 50, 52 ), each of which is provided with a closing device ( 60, 62, 64 ). The catalytic reactor ( 16 ) is operative for causing said process gas (F) to flow through a first catalyst bed segment ( 48 ). Said process gas (F) is at a first temperature at which the sulphur trioxide that is entrained in said hot process gas is at least partially precipitated out on to the catalytic material that said first catalyst bed segment ( 48 ) embodies. Periodically said closing device ( 60 ) is operated in order to thereby isolate said first bed segment ( 48 ) from the flow therethrough of said hot process gas (F). A regeneration system ( 34, 36, 38 ) is also provided that is operative for purposes of causing a regenerating gas to flow through the first bed segment ( 48 ). In addition, a sulphur trioxide removal device ( 20 ) is provided, which is separate from said catalytic reactor ( 16 ), and which is operative for purposes of effecting therewith the removal of the sulphur trioxide from said regenerating gas.

CROSS REFERENCE TO RELATED APPLICATIONS

This is a divisional application claiming priority to pendingapplication Ser. No. 12/205,135 having a filing date of Sep. 5, 2008,incorporated herein in its entirety by reference.

FIELD OF THE INVENTION

The present invention relates to a method of removing, at leastpartially, nitrogen oxides from a hot process gas through the use of acatalytic reactor that embodies catalytically active material, andwherein said catalytic reactor includes a catalyst bed comprising atleast two catalyst bed segments that are arranged in parallel relationwith respect to the direction of flow of the hot process gas, each ofsaid at least two catalyst bed segments being provided with a closingdevice, such that each of said at least two catalyst bed segments can beindividually isolated from the flow of the hot process gas.

The present invention further relates to a gas cleaning system which isoperative for removing, at least partially, nitrogen oxides from a hotprocess gas.

BACKGROUND OF THE INVENTION

During the combustion of a fuel, such as coal, oil, peat, waste, etc.,in a combustion plant, such as a power plant, a hot process gas isgenerated, and such hot process gas contains, among other components,nitrogen oxides, usually denoted as NOx, and sulphur oxides, usuallydenoted as SOx. The nitrogen oxides are mainly comprised of nitrogenmonoxide, denoted as NO, and nitrogen dioxide, denoted as NO₂, and thesulphur dioxides are mainly comprised of sulphur dioxide, denoted asSO₂, and sulphur trioxide, denoted as SO₃, the amount of sulphurtrioxide, SO₃, normally constituting less than 5% of the total amount ofSOx. The removal of NOx is usually accomplished through the use of aso-called Selective Catalytic Reduction (SCR) process. In accordancewith such a process, NOx is reduced by means of ammonia gas, NH₃, toform nitrogen gas, N₂, in the presence of a catalytically activematerial. Such a catalytically active material commonly comprises metaloxides, such as, by way of exemplification, vanadium pentoxide, denotedas V₂O₅, and tungsten trioxide, denoted as WO₃.

A problem associated with the use of many of such catalytically activematerials is that they after a period of time become contaminated bydeposits of sulphur trioxide, SO₃, thereon that tend to form ammoniumsulphates, which results in a decrease in the nitrogen oxide removalefficiency of these catalytically active materials. The normal procedurethat is employed for purposes of overcoming such problems is to operatea reactor containing such catalytically active material at a rather hightemperature, usually above 300° C., in an attempt to prevent the sulphurtrioxide, SO₃, from precipitating out on to the catalytically activematerial.

In U.S. Pat. No. 5,762,885 there is described an oxidation catalystabsorber, which embodies a number of catalyst segments arranged inparallel relation and comprising platinum or palladium. Each of thesecatalyst segments is provided with two louvers, such that each one ofthe respective catalyst segments can be “closed” off insofar as theprocess gas that is to be cleaned is concerned. Moreover, the catalystsegments that are closed off can be regenerated through the use of aregenerating gas that contains hydrogen gas, the latter being operativeto remove pollutants from the catalytic material embodied by suchcatalyst segments that are closed off. After being spent, suchregenerating gas is then recycled to the reactor, where the spentregenerating gas is mixed with the process gas at a point locatedupstream of the catalyst bed.

While a catalyst absorber such as that described in U.S. Pat. No.5,762,885 might be deemed to be effective for purposes of effecting thecleaning of a process gas that is generated in a natural gas firedturbine power plant, wherein the concentration of SOx in the process gasis very low, i.e., generally lower than 1 ppm, such a catalyst absorberis not suitable for cleaning process gas, e.g., flue gases, in which theconcentration of SOx is higher than about 5 ppm. In a process gas thatis generated during the combustion of coal or oil, or during theincineration of waste, the concentration of SOx in such a process gas isoften in the range of 10 to 5000 ppm. Furthermore, the type of catalyst,which is employed, that is, an oxidation catalyst that contains noblemetals like platinum or palladium, necessitates that there only be anextremely low concentration of catalyst pollutants, such as, by way ofexemplification, mercury, lead and other heavy metals, in the hotprocess gas. Such a requirement restricts the use of the reactor towhich U.S. Pat. No. 5,762,885 is directed to so-called “clean fuels”,such as natural gas. For plants in which coal, oil, peat, waste, etc,are combusted the reactor to which U.S. Pat. No. 5,762,885 is directedwould not be capable of providing an acceptable level of nitrogen oxideremoval efficiency.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a method of cleaning ahot process gas, which method is effective for removing NOx and SOx, andin particular sulphur trioxide, SO₃, from the hot process gas both whenthe hot process gas contains high concentrations of SOx and when the hotprocess gas contains low concentrations of SOx.

This object is achieved by means of a method of removing, at leastpartially, nitrogen oxide from a hot process gas through the use of acatalytic reactor that embodies catalytically active material, andwherein said catalytic reactor includes a catalyst bed comprising atleast two catalyst bed segments that are suitably arranged in parallelrelation with respect to the direction of flow of the process gas, eachof said at least two catalyst bed segments being provided with a closingdevice, such that each of said at least two catalyst bed segments can beisolated from the flow of the hot process gas, said method beingcharacterized in that the steps thereof comprise the following:

passing said hot process gas through at least a first bed segment ofsaid at least two catalyst bed segments, wherein said hot process gas isat a first temperature at which the sulphur trioxide, SO₃, that isentrained in the hot process gas is at least partially precipitated onto the catalytic material that the first bed segment embodies,

periodically operating the closing device of said first bed segment inorder to thereby isolate said first bed segment from the flow of hotprocess gas, while at least one second bed segment of said at least twocatalyst bed segments remains operative for purposes of removing fromsaid hot process gas the sulphur trioxide, SO₃, and the nitrogen oxidethat is entrained therein,

causing a regenerating gas to flow through said first bed segment whensaid first bed segment is isolated from the flow of hot process gas, and

then causing said regenerating gas, after said regenerating gas hasflowed through said first bed segment, to flow through a sulphurtrioxide, SO₃, removal device that is separate from said catalyticreactor.

An advantage of this method of the present invention is that thecatalyst bed is designed to be operative for purposes of removing boththe nitrogen oxides and the sulphur trioxide, SO₃, that are entrained inthe process gas. This is accomplished by virtue of the sulphur trioxide,SO₃, being permitted to condense on to the catalytic material that thecatalyst bed embodies. Contrary to the method of the present invention,the methods of the prior art are characterized by the fact that they arefocused on avoiding the precipitation of the sulphur trioxide, SO₃, thatis entrained in the process gas on to the catalytic material that thecatalyst bed embodies, since such precipitation of such sulphurtrioxide, SO₃, thereon results in a reduction in the efficiency that iscapable of being realized with the catalytic material.

In accordance with the method of the present invention, the catalyst bedsegments are regenerated during the normal operation of the catalyticreactor, such that no shut-down or interruption of the operation of thecatalytic reactor is therefore required. To this end, the regeneratinggas after flowing through the catalyst bed segments is made to flow fromthe catalytic reactor to a separate device, which is operative forpurposes of capturing therein the sulphur trioxide, SO₃, that isentrained in the regenerating gas. By virtue of the use of such aprocess the sulphur trioxide is first effectively removed from theprocess gas through the operation of the catalytic reactor, and is thenremoved from the catalytic reactor, by virtue of the flow therethroughof regenerating gas, and is finally disposed of in the sulphur trioxide,SO₃, removal device by virtue of the operation thereof, with the sulphurtrioxide, SO₃, removal device being located separately from thecatalytic reactor.

According to one embodiment of the method of the present invention, saidregenerating gas is at a second temperature, which is higher than saidfirst temperature. An advantage of this embodiment of the presentinvention is that a regenerating gas that is at a second temperature,which is higher than the first temperature, the latter temperature beingthe temperature of the hot process gas, causes the regeneration of thecatalyst bed segments to be more effective and to occur quicker.

According to one embodiment of the method of the present invention saidfirst temperature is in the range of 180-300° C., and said secondtemperature is at least 310° C. Said first temperature, being thetemperature of the process gas passing through the catalytic reactor,which is in the range of 180-300° C. has been found to be effectivesince such a temperature provides for both an effective reduction ofnitrogen oxides, and an effective condensation of the sulphur trioxide,SO₃, on to the catalytic material that the catalyst bed segments embody.Said second temperature, being the temperature of the regenerating gas,of at least 310° C. has been found to provide for an efficientevaporation of sulphur trioxide, SO₃, from the catalyst material thatthe catalyst bed segments embody. Preferably, in accordance with themethod of the present invention said second temperature should be lessthan about 400° C., because temperatures higher than this increases themechanical strain both on the mechanical components and on the catalyticmaterial, and increases the energy costs as well.

According to one embodiment of the method of the present invention, saidsulphur trioxide, SO₃, removal device preferably is selected from thegroup of devices that encompasses, by way of exemplification, wetscrubbers, dry scrubbers, fabric filters, and electrostaticprecipitators. All of these devices are commonly utilized in gascleaning systems, and are all suitable for purposes of effectingtherewith the final disposal of sulphur trioxide, SO₃, which has beenremoved from the catalytic reactor by virtue of the flow therethrough ofthe regenerating gas.

According to one embodiment of the method of the present invention, saidsulphur trioxide, SO₃, removal device is located at a point downstreamof the catalytic reactor with respect to the direction of flow of thehot process gas, such as to be operative for purposes of removing fromthe hot process gas and from the regenerating gas the sulphur speciesthat may be entrained therein. An advantage of this embodiment of themethod of the present invention is that the sulphur trioxide, SO₃,removal device is operative to perform the dual purpose of removing thesulphur dioxide from the hot process gas, and of capturing as well thesulphur trioxide, SO₃, that has been removed by operation of theregenerating gas. As a consequence thereof, this reduces both theinvestment and maintenance costs that are associated with the use of thegas cleaning system.

According to one embodiment of the method of the present invention, saidregenerating gas is cooled after having been passed through said firstbed segment and before said regenerating gas has been made to flowthrough said sulphur trioxide, SO₃, removal device. An advantage of thisembodiment of the present invention is that the sulphur trioxide, SO₃,is at least partially condensed to form a liquid solution, which makesit easier to handle the sulphur trioxide, SO₃, during the furtherprocessing thereof.

According to one embodiment of the method of the present invention, saidregenerating gas is caused, after said regenerating gas has been passedthrough said first bed segment, to flow through a sulphur trioxide, SO₃,removal device whereby at least a portion of said regenerating gas ismixed with an absorption medium that is being circulated in said sulphurtrioxide, SO₃, removal device for the purpose of effecting as a resultthereof the removal of the sulphur dioxide from said hot process gas. Anadvantage of this embodiment of the method of the present invention isthat said sulphur trioxide, SO₃, is able to react directly with theabsorption medium so as to thereby form a product, such as gypsum, whichis easy both to handle and to dispose of.

According to one embodiment of the method of the present invention, saidregenerating gas preferably is selected from the group of gases thatencompasses, by way of exemplification, steam, air, nitrogen, flue gas,and mixtures thereof. An advantage derived from the utilization of suchgases is that such gases are often readily available at a combustionplant, and are so available thereat at a reasonable cost. Furthermore,such gases are non-combustible, and therefore are easily handled.

A further object of the present invention is to provide a gas cleaningsystem, which is effective for purposes of removing from a hot processgas both NOx and SOx, and in particular sulphur trioxide, SO₃, therefromboth when said hot process gas has entrained therein high concentrationsas well as low concentrations of SOx.

Such an object is achieved in accordance with the present inventionthrough a gas cleaning system that is adapted for purposes of removing,at least partially, nitrogen oxides from a hot process gas, said gascleaning system includes a catalytic reactor embodying catalyticallyactive material, said catalytic reactor in turn includes a catalyst bedcomprising at least two catalyst bed segments that are suitably arrangedin parallel relation with respect to the direction of flow of the hotprocess gas, with each of said at least two catalyst bed segments havinga closing device associated therewith, such that each of said at leasttwo catalyst bed segments can be individually isolated from the flow ofthe hot process gas, the gas cleaning system in accordance with thepresent invention being characterized in that said catalytic reactor isdesigned to be operative for purposes of causing said hot process gas toflow through at least a first bed segment of said at least two catalystbed segments, and with said hot process gas being at a first temperatureat which sulphur trioxide, SO₃, entrained in the hot process gas is atleast partially precipitated on to the catalytic material that the firstbed segment embodies, and for periodically operating said closing deviceto thereby isolate said first bed segment from the flow of hot processgas, while at least one second bed segment of said at least two catalystbed segments remains operative for purposes of removing from said hotprocess gas the sulphur trioxide, SO₃, and the nitrogen oxide that isentrained therein, said gas cleaning system further comprises aregeneration system, which is designed to be operative for purposes ofcausing a regenerating gas to flow through said first bed segment whensaid first bed segment is isolated from the flow of hot process gas, aswell as a sulphur trioxide, SO₃, removal device that is separate fromsaid catalytic reactor and is designed to be operative for purposes ofeffecting therewith the removal from said regenerating gas of thesulphur trioxide, SO₃, that is entrained therein after said regeneratinggas has flowed through said first bed segment.

An advantage of the gas cleaning system of the present invention is thatthe catalytic reactor is designed to be operative for purposes ofeffectively removing from the process gas both the nitrogen oxides andthe sulphur trioxide, SO₃, that are entrained in the process gas. Assuch, the gas cleaning system requires few components.

Further objects and features of the present invention will be apparentfrom the following description thereof when considered along with theillustration thereof in the drawings and as defined in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail withreference to the appended drawings in which:

FIG. 1 is a schematic side view of a power plant with which the presentinvention is capable of being employed.

FIG. 2 is a schematic side view of a catalytic reactor that is designedto be employed with the present invention.

FIG. 3 is a schematic side view of the catalytic reactor of FIG. 2 whenthe latter is being operated in a cleaning mode.

FIG. 4 is a schematic side view of a wet scrubber that is capable ofbeing employed with the present invention.

FIG. 5 is a schematic side view of a dry scrubber that is capable ofbeing employed with the present invention.

DESCRIPTION OF PREFERRD EMBODIMENTS

FIG. 1 is a schematic side view in which there is illustrated a powerplant 1, as seen from the side thereof. The power plant 1 includes aboiler 2. During the combustion of a fuel, such as coal or oil, a hotprocess gas, often referred to by those in the industry as a flue gas,is generated in the boiler 2. This flue gas exits from the boiler 2 viaa gas duct 4. The gas duct 4 in turn is connected to an air preheater 6.The air preheater 6 is designed to be operative for purposes ofeffecting the heating of the combustion air that is to be supplied tothe boiler 2 via an air duct 8. A gas duct 10 is operative for purposesof causing the flue gas, which is generated in the boiler 2, to flowfrom the air preheater 6 to an electrostatic precipitator 12. Theelectrostatic precipitator 12 is designed to be operative for purposesof removing from the flue gas that is made to flow thereto the dustparticles that are entrained in said flue gas. An example, by way ofexemplification and not limitation, of an electrostatic precipitator canbe found described and illustrated in U.S. Pat. No. 4,502,872, which ishereby incorporated herein by virtue of this reference thereto.

Continuing, a gas duct 14 is provided that is designed to be operativefor purposes of causing the flue gas, from which most of the dustparticles that are entrained therein have been removed therefrom, toflow from the electrostatic precipitator 12 to a catalytic reactor 16.The flue gas entering the catalytic reactor 16 typically is at atemperature in the range of 180-300° C. The amount of the concentrationof sulphur dioxide, SO₂, that is entrained with the flue gas as the fluegas enters the catalytic reactor 16 typically would be in the range of10-5000 ppm of SO₂. Whereas, the amount of the concentration of sulphurtrioxide, SO₃, that is entrained with the flue gas as the flue gasenters the catalytic reactor 16 typically would be in a range of 1-50ppm SO₃. As will be described in more detail hereinafter, the catalyticreactor 16 is designed to be operative for purposes of removing from theflue gas both the nitrogen oxides, NOx, and the sulphur trioxide, SO₃,that are entrained with the flue gas through the use of one and the sametype of catalyst material. To this end, the catalytic reactor 16typically is capable of removing about 60-95% of the nitrogen oxides,NOx, entering the catalytic reactor 16, and typically is also capable ofremoving about 30-90% of the sulphur trioxide, SO₃, entrained with theflue gas entering the catalytic reactor 16.

With further reference to FIG. 1, as illustrated therein, a gas duct 18is provided that is designed to be operative for purposes of causing theflue gas, from which most of the nitrogen oxides, NOx, and at least someof the sulphur trioxide, SO₃, that has been removed therefrom, is madeto flow from the catalytic reactor 16 to a bubbling bed type of wetscrubber 20. An example, by way of exemplification and not limitation,of such a bubbling bed type of wet scrubber can be found described andillustrated in WO 2005/007274, which is hereby incorporated herein byvirtue of this reference thereto. In the bubbling bed type of wetscrubber 20, the flue gas is made to flow through an apertured plate 22,and then through a layer 24 of an absorption liquid that is made to flowover the apertured plate 22. Said absorption liquid preferably comprisesa limestone slurry, which is designed to react with sulphur dioxide thatis entrained with the flue gas in order to thereby form calciumsulphite, which can be oxidized to form gypsum, the latter being capableof being easily disposed of. Continuing with the description of thepower plant 1 a pump 26 is provided that is designed to be operative forpurposes of circulating the absorption liquid in the wet scrubber 20 bymeans of a pipe 28, which is operative to supply the absorption liquidto effect therewith the formation of the layer 24 of the absorptionliquid.

Continuing, as best understood with reference to FIG. 1, a gas duct 30is operative for purposes of causing the flue gas, from which thesulphur dioxide has been removed therefrom, to flow from the wetscrubber 20 to a stack 32, from which the flue gas from which thesulphur dioxide has been removed is released to the atmosphere.

The process of removing from the flue gas the nitrogen oxides, NOx, andthe sulphur trioxide, SO₃, in the catalytic reactor 16 involves the useof a regenerating gas, as will be described in more detail hereinafter.Such a regenerating gas can comprise steam, which is at a suitabletemperature, that is preferably at a temperature of more than 310° C. Tothis end, the temperature of the regenerating gas preferably is lessthan about 400° C. , since any higher temperature would be operative toproduce an increased mechanical strain on the mechanical components ofthe catalytic reactor 16, and might also produce a negative effect onthe catalytic material of the catalytic reactor 16. Often a regeneratinggas temperature of about 315 to 350° C. is suitable to achieve therewithan efficient regeneration of the catalytic material of the catalyticreactor 16.

With further reference to the illustration of the power plant 1, a firststeam pipe 34 is provided that is designed to be operative for purposesof causing the regenerating gas, which is in the form of steam, to flowfrom a regenerating gas supply system, which is schematicallyillustrated in FIG. 1 as a steam tank 36, to the catalytic reactor 16.There is also provided a second steam pipe 38 that is designed to beoperative for purposes of causing steam to flow from the catalyticreactor 16 to a cooler, which preferably in accordance with the presentinvention is in the form of a condenser 40. In addition, a coolingcircuit 42 is provided that is designed to be operative for purposes ofsupplying a cooling medium, such as cold water, to the condenser 40. Thecondenser 40 is designed to be operative so that the steam is caused tobe cooled to less than about 90° C., which results in most of the steamthat is made to flow thereto as well as what may be entrained thereincondensing. It is believed that the regenerating gas leaving thecatalytic reactor 16 contains gaseous sulphur trioxide, SO₃, andammonia, NH₃, that may react, at least partly, to form, due to the lowtemperature in the condenser 40, solid ammoniumbisulfate, NH₄HSO₄, suchsolid compound being easily captured in the liquid of the condensor 40.Furthermore, a condensate pipe 44 is provided that is designed to beoperative for purposes of causing the condensate, which is formed in thecondenser 40, to flow to the pipe 28, the latter pipe 28, as describedpreviously hereinbefore, in turn being operative to effect thecirculation of the absorption liquid of the wet scrubber 20. To thisend, the condensate of the condenser 40 thus is mixed with theabsorption liquid of the wet scrubber 20, such that at least some of thesulphur compounds of the condensate, such as the sulphur trioxide, SO₃,react with the limestone that is present in the absorption liquid tothereby form calcium compounds, such as, for example, gypsum.

Hence, and as will be described in more detail hereinafter, thecatalytic reactor 16 is thus operative for purposes of effectingtherewith the removal from the flue gas of both the nitrogen oxides,NOx, and the sulphur trioxide, SO₃, that is entrained with the flue gas.Thereafter, the sulphur trioxide is then removed from the catalyticreactor 16 by virtue of the flow through the catalytic reactor 16 of aregenerating gas. Said regenerating gas is then cooled in the condenser40 to thereby effect the condensation of the sulphur trioxide that isentrained with the regenerating gas. The condensate that is produced inthe condenser 40 is then mixed with the absorption liquid that flows inthe wet scrubber 20 in order to thereby form, for example, gypsum. Thewet scrubber 20 is also operative to effect the removal from the fluegas of the sulphur dioxide, SO₂, that is entrained with the flue gas asthe flue gas flows through the catalytic reactor 16. Thus, all of thesulphur species that are entrained with the flue gas finally end up inthe wet scrubber 20, from which these sulphur species may be removed inthe form of gypsum.

However, the wet scrubber 20, as such, has been deemed not to besuitable for use for purposes of for removing sulphur trioxide, SO₃,from a flue gas. Basically, the reason for this is believed to be thatthe sulphur trioxide, SO₃, when still entrained with the flue gas uponentering the wet scrubber 20, tends to form a sulphur trioxide, SO₃,aerosol comprising very small aerosol droplets, which are not capable ofbeing effectively removed in the wet scrubber 20. On the other hand,through the use of the process of the present invention, which will bedescribed in further detail hereinafter, the sulphur trioxide, SO₃, iscapable of being removed in the catalytic reactor 16, after which thesulphur trioxide, SO₃, in the form of a condensate is designed to bemixed directly with the absorption liquid that is in the wet scrubber20, such that no sulphur trioxide, SO₃, aerosol is thus formed.

FIG. 2 is a schematic side view illustrating in more detail a portion ofthe catalytic reactor 16 that is designed to be employed with thepresent invention. The catalytic reactor 16, as best understood withreference to FIG. 2, includes a horizontal catalyst bed layer 46, whichpreferably consists of three catalyst bed segments; namely, a firstcatalyst bed segment 48, a second catalyst bed segment 50, and a thirdcatalyst bed segment 52. These three catalyst bed segments 48, 50, 52are suitably arranged in parallel relation with respect to the directionof flow of the flue gas, the latter direction being illustrated by meansof the arrow denoted by the letter F in FIG. 2. The flue gas F, asdescribed hereinbefore previously, is at a temperature that is typicallyin the range of 180-300° C. Each of the catalyst bed segments 48, 50, 52embodies a catalytically active material, such as, by way ofexemplification and not limitation, vanadium pentoxide, V₂O₅, and/ortungsten trioxide, WO₃. Such a catalytically active material is designedto be supported on a carrier structure, such as, by way ofexemplification and not limitation, a ceramic honeycomb or a platestructure, such as, for example, a titanium dioxide structure. Suchtitanium dioxide structures are known in the prior art technologieswherein they have been employed in selective catalytic reduction (SCR)devices for purposes of effecting therewith the removal of nitrogenoxides.

Continuing, a source of ammonia, schematically illustrated in FIG. 2 inthe form of an ammonia tank 54, is designed to be operative for purposesof supplying gaseous ammonia via a pipe system 56 preferably to aplurality of nozzles 58. In turn, the nozzles 58 are designed to beoperative to cause the ammonia to be thoroughly mixed with the flue gasF. As such, thereafter, when passing through the catalyst bed segments48, 50, 52 the nitrogen oxides are reduced through the interactionthereof with the ammonia thereby resulting in the production of nitrogengas in accordance with principles that are well-known to those skilledin the art of the selective catalytic reduction (SCR) of nitrogenoxides. An example of such a SCR principle, and the reactions betweenthe nitrogen oxides and ammonia that result from the application of sucha principle, can be found described and illustrated in U.S. Pat. No.5,555,849, the teachings of which are hereby incorporated herein byvirtue of this reference thereto. As an alternative to the use ofammonia it is equally possible to utilize urea in lieu of ammonia.

Each of the catalyst bed segments 48, 50, 52 is designed to be providedwith a closing device 60, 62, 64, respectively. Each such closing device60, 62, 64, in accordance with the present invention, comprises an inletdamper 66 and an outlet damper 68. Each of the closing devices 60, 62,64 are designed to be operative to effect therewith the isolation of therespective one of the catalyst bed segments 48, 50, 52 from the flow ofthe flue gas F. As illustrated in FIG. 2, all of the closing devices 60,62, 64 are depicted as being in an open position, and the flow of theflue gas F is, as such, divided into three substantially equal partialflows, which are denoted in FIGS. 2 as F1, F2, and F3, respectively thatflow through the respective one of the catalyst bed segments 48, 50, 52in a vertical direction as viewed with reference to FIG. 2.

As they flow through the respective catalyst bed segments 48, 50, 52,the nitrogen oxides are reduced through the interaction thereof with theammonia that is supplied thereto, as has been described hereinbeforepreviously. Furthermore, a substantial fraction, typically on the orderof 30-90%, of the sulphur trioxide, SO₃, content of the flue gas F isdeposited on to the catalytic material that the catalyst bed segments48, 50, 52 each embody. The reason that such a deposition occurs is thatthe temperature of the flue gas F is comparatively low, which in turnresults in the condensation of at least a portion of the sulphurtrioxide, SO₃. Continuing, the catalyst material embodied by each of thecatalyst bed segments 48, 50, 52 consists of a large specific area,which is typically defined as being a so-called BET-area, that as suchis operative to provide numerous active sites at which it is possiblefor the reaction between the nitrogen oxides and the ammonia to occur.Typically, such a BET-area, when measured in accordance with theprovisions of ASTM C1274-00 (2006), would be at least 20 m²/g, andpreferably at least 50 m²/g. Such a large specific area is thusoperative to provide a suitable surface at which the condensation of thesulphur trioxide, SO₃, can take place. Hence, the catalyst bed segments48, 50, 52 are thus operative to reduce the nitrogen oxides inaccordance with principles of selective catalytic reduction (SCR), andto cause the capture of the sulphur trioxide, SO₃, as a consequence ofthe occurrence of condensation reactions. As such, the flue gas thatleaves the catalyst bed segments 48, 50, 52, in the form of the threepartial flows that are denoted in FIGS. 2 as C1, C2, and C3, issubstantially clean, insofar as the amount of the concentration of thenitrogen oxides and the sulphur trioxide, SO₃, entrained therewith isconcerned.

After the operation thereof for a sufficient period of time, thecatalyst bed segments 48, 50, 52 will have captured such an amount ofsulphur trioxide, SO₃, that the selective catalytic reduction ofnitrogen oxides will be negatively effected thereby. For this reasonthere is provided the steam tank 36. To this end, the first steam pipe34, which has been described hereinbefore previously with reference tothe power plant 1 that is illustrated in FIG. 1 is comprised of threeseparate pipes 70, 72, 74. Each of these pipes 70, 72, 74 is suitablyprovided with a steam valve 76. Furthermore, the second steam pipe 38,which has been described hereinbefore previously with reference to thepower plant 1 that is illustrated in FIG. 1, is connected to threeseparate pipes 78, 80, 82, each of which is suitably provided with asteam valve 84. In accordance with the illustration thereof in FIG. 2,all of the steam valves 76, 84 are depicted therein as being in a closedposition.

FIG. 3 is a side view, similar to that of FIG. 2, and depicts thecatalytic reactor 16 as being in a cleaning mode of operation. When insuch a cleaning mode the inlet damper 66 and the outlet damper 68 of theclosing device 60 are each in a closed position, such as to therebyeffect the isolation of the first bed segment 48 from the flow of theflue gas F. As a result thereof, the entire flow of the flue gas F flowsthrough the second and third bed segments 50, 52 in the form of theflows of flue gas, which are denoted as F2 and F3 in FIG. 3. Withfurther reference to FIG. 3, the valve 76 of the pipe 70 is depictedtherein as being in an open position, and so is the valve 84 of the pipe78 depicted therein as being in an open position. Thus, as a result ofthis steam, at a temperature of at least 310° C., is caused to flow fromthe steam tank 36, via the pipe 70 to the first bed segment 48 andtherethrough. By virtue of the high temperature of such steam thesulphur trioxide, SO₃, that has been condensed on to the catalystmaterial of the first catalyst bed segment 48 is caused to evaporate.Thereafter, the steam together with the sulphur trioxide, SO₃, that hasbeen evaporated is evacuated via the pipes 78 and 38 from the first bedsegment 48 and is then transmitted to the condenser 40, to whichreference has been made hereinbefore previously in connection with thedescription of the power plant 1 that is illustrated in FIG. 1. Thus,the steam from the steam tank 36 is operative to thermally regeneratethe catalytic material that the first bed segment 48 embodies in orderto thereby effect a restoration of its capability of reducing of thenitrogen oxides, NOx, by virtue of the removal of the sulphur trioxide,SO₃, from the catalytic material. As described hereinbefore previouslywith reference to the description of the power plant 1 that isillustrated in FIG. 1, the steam with which the sulphur trioxide, SO₃,is entrained is cooled in the condenser 40 whereby a condensate isformed after which such condensate is fed to a sulphur trioxide, SO₃,removal device in the preferred form of the wet scrubber 20 inaccordance with the present invention.

As will be best understood from a reference to FIG. 3, the second andthird catalyst bed segments 50, 52, in accordance with the illustrationthereof in FIG. 3, remain in operation and are thus operative to reducethe content of nitrogen oxides of the entire flow of the flue gas F.After the first catalyst bed segment 48 has been regenerated in themanner that has been described in the previous paragraph, the valve 76of the pipe 70 and the valve 84 of the pipe 78 are both closed, andconcomitantly the closing device 60 is opened. Next, the second catalystbed segment 50 can then be regenerated by being isolated from the flowof flue gas F by virtue of the operation of the closing device 62.Accordingly, the cleaning of any one of the catalyst bed segments 48,50, 52 can be accomplished while the catalytic reactor 16 is stillon-line and remains in an operational mode. It will be appreciated thatwithout departing from the essence of the present invention the totalnumber of catalyst bed segments can be designed such as to ensure thatthose catalyst bed segments that remain in operation are sufficient tohandle the entire flow of the flue gas F while one or more of thecatalyst bed segments have been isolated from the flow of the flue gas Ffor the purpose of enabling the catalyst bed segments, which have beenisolated, to undergo a cleaning sequence in accordance with what hasbeen described hereinbefore with reference to the structure that isillustrated in FIG. 3.

FIG. 4 illustrates an alternative embodiment of a sulphur trioxide, SO₃,removal device that is in the form of a wet scrubber tower 120. A wetscrubber tower, such as, by way of exemplification, the wet scrubbertower 120 that is depicted in FIG. 4, is previously known fromapplications wherein sulphur dioxide, SO₂, is designed to be removedfrom a flue gas. In this regard, reference may be had, by way ofexemplification and not limitation, to, for example, EP 162 536, theteachings of which are hereby incorporated herein by virtue of thisreference thereto. The wet scrubber tower 120 includes a gas inlet 122and a gas outlet 123. The flue gas enters the gas inlet 122, via theduct 18 to which reference has been had hereinbefore previously inconnection with the description of the power plant 1 that is illustratedin FIG. 1, and is then caused to flow vertically upwards, as viewed withreference to FIG. 4, through a cylindrical portion 125 of the wetscrubber tower 120, in the manner that is illustrated by means of thearrow that is denoted by the letter F in FIG. 4. The lower portion 127of the wet scrubber tower 120, as best understood with reference to FIG.4, is shaped in the form of a tank, which contains a limestone basedabsorption medium that preferably is in the form of a liquid slurry.Continuing, a pump 126 is provided for purposes of effecting therewiththe pumping of the absorption liquid, via a pipe 128, to a number ofnozzles 129. The nozzles 129 in turn are operative to inject said liquidslurry in to the flue gas F, in order to effect therewith the removal ofsulphur dioxide, SO₂, from the flue gas F. Accordingly, a flue gas thatis substantially pure in nature exits from the outlet 123 and is thusmade to flow therefrom to the stack. Just as in the case of the wetscrubber 20, which has been described hereinbefore previously inconnection with the description of the power plant 1 that is illustratedin FIG. 1, the sulphur dioxide, SO₂, that is removed from the flue gasforms gypsum, which can easily be readily disposed of.

The condensate that is formed in the condenser 40, to which referencehas been made hereinbefore previously in connection with the descriptionof the power plant 1 that is illustrated in FIG. 1, may be added withoutdeparting from the essence of the present invention to the lower portion127 of the wet scrubber tower 120 via a pipe 144. If this is done, thecondensate in the lower portion 127 of the wet scrubber tower 120 isthen mixed with the liquid slurry, such that the sulphur trioxide, SO₃,in the condensate forms gypsum, in the manner that has been describedhereinbefore previously in connection with the description of the powerplant 1 that is illustrated in FIG. 1. Accordingly, the wet scrubbertower 120 is thus operative for purposes of effecting the removal of thesulphur dioxide, SO₂, from the flue gas F that has been made to flowthrough the catalytic reactor 16, and concomitantly is operative toeffect the capture of the sulphur trioxide, SO₃, that has been removedfrom the catalytic reactor 16 by the regenerating gas, i.e., the steam,that is made to flow through the catalytic reactor 16.

In FIG. 5 there is illustrated yet a further alternative embodiment of asulphur trioxide, SO₃, removal device, which as illustrated therein isin the form of a dry scrubber 220. A dry scrubber, such as, by way ofexemplification, the dry scrubber 220 that is depicted in FIG. 5, ispreviously known from applications wherein sulphur dioxide, SO₂, isdesigned to be removed from a flue gas. In this regard, reference may behad, by way of exemplification and not limitation, to, for example, WO2004/026443, the teachings of which are hereby incorporated herein byvirtue of this reference thereto. Continuing, the dry scrubber 220includes a contact reactor 222 in which the flue gas F is brought intocontact with a dry, but moistened, absorption medium that preferably isin the form of a dry powder. Such a dry powder could, by way ofexemplification, consist of a mixture of hydrated lime and the reactionproducts that are produced during the reaction between such hydratedlime and the sulphur dioxide, SO₂, that is entrained in the flue gas F.This dry powder is then in turn collected in a fabric filter, i.e., atextile filter, which preferably is in the form of a bag house 224. Anexample of such a bag house can be found described and illustrated, byway of exemplification and not limitation, in U.S. Pat. No. 4,336,035,the teachings of which are hereby incorporated herein by virtue of thisreference thereto. Continuing, a first portion of such dry powder thatis so collected in the bag house 224 preferably is removed for purposesof the disposal thereof. Whereas, a second portion of such dry powder isrecirculated to a mixer 226. It is in the mixer 226, the latterconsisting of a fluidised bed, which is supplied with compressed air viaa pipe 227, that the recirculated dry powder is then mixed with bothfresh hydrated lime that is supplied via a pipe 229 and water that issupplied via a pipe 231. From the mixer 226, the mixture that has beenformed therein, that is, the absorption medium, is made to flow to thecontact reactor 222. With further reference to FIG. 5, as bestunderstood with reference thereto, the condensate from the condenser 40,to which reference has been had hereinbefore previously with regard tothe power plant 1 that is illustrated in FIG. 1, is supplied to themixer 226 via a pipe 244. In the mixer 226, the sulphur trioxide, SO₃,that is entrained with the condensate that is made to flow theretoreacts with the hydrated lime thereby forming gypsum. To this end, thedry scrubber 220 is thus operative to effect the removal of the sulphurdioxide from the flue gas F that has been made to flow through thecatalytic reactor 16, and concomitantly is operative to effect thecapture of the sulphur trioxide, SO₃, that has been removed from thecatalytic reactor 16 by the regenerating gas, i.e., the steam, that ismade to flow through the catalytic reactor 16.

It will be appreciated that without departing from the essence of thepresent invention numerous variants of the embodiments described aboveare possible within the scope of the appended claims.

For example, by way of exemplification and not limitation, it has beendescribed hereinbefore previously that the gas utilized for regeneratingthe catalyst bed segments preferably is steam at a temperature of atleast 310° C. However, it will be appreciated that other gases canequally well also be utilized for purposes of effecting the regenerationof the catalyst bed segments without departing from the essence of thepresent invention. For instance, air, nitrogen gas, or combustion gasfrom a natural gas or a similarly clean fuel can equally well also beutilized as a regenerating gas without departing from the essence of thepresent invention. In accordance with a further alternative of thepresent invention, it is possible without departing from the essence ofthe present invention to utilize a flue gas as the regenerating gas,preferably a cleaned flue gas from which most of the particulates,nitrogen oxides and sulphur species have previously been removed.Further to this point, such flue gases are preferably also heated to atemperature of at least 310° C. to thereby enable an efficient and quickregeneration of the catalyst bed segments to be had therewith. Since theregeneration of the catalyst bed segments is based on thermally removingthe sulphur trioxide, SO₃, therefrom, the regenerating gas as such doesnot need to embody any hydrogen gas. To this end, the regenerating gasin fact preferably is substantially hydrogen-free, i.e., contains lessthan about 500 ppm of hydrogen gas, since a substantial amount ofhydrogen gas in the regenerating gas would potentially pose possiblerisks of fire and explosion.

It has been described previously hereinbefore that alternatively a wetscrubber 20, a wet scrubber tower 120, or a dry scrubber 220 may beutilized without departing from the essence of the present invention forpurposes of effecting therewith the removal of sulphur dioxide, SO₂,from the flue gas and for purposes of effecting the capture of sulphurtrioxide, SO₃, that has been removed from the catalytic reactor 16 byvirtue of the passage therethrough of the regenerating gas. It will beappreciated that other devices may equally well also be utilized assulphur trioxide, SO₃, removal devices without departing from theessence of the present invention, such as, by way of exemplification andnot limitation, electrostatic precipitators and fabric filters that donot necessarily constitute a part of a dry scrubber. A still furtheralternative is to utilize a spray drying absorber for purposes ofeffecting therewith the removal of sulphur dioxide, SO₂, from the fluegas and for purposes of effecting the capture of sulphur trioxide, SO₃,that has been removed from the catalytic reactor 16 by virtue of thepassage therethrough of the regenerating gas. In this regard, referencemay be had, by way of exemplification and not limitation, to, forexample, U.S. Pat. No. 4,755,366, the teachings of which are herebyincorporated herein by virtue of this reference thereto. In such a spraydrying absorber the condensate from the condenser, containing thesulphur trioxide, SO₃, could be mixed with an aqueous suspension that isthen supplied to an atomiser of the spray drying absorber.

The sulphur trioxide, SO₃, removal device may also, as yet anotheralternative, be in the form, without departing from the essence of thepresent invention, of a separate unit, that in no way has anything to dowith the removal of sulphur dioxide, SO₂, from the flue gas.

The catalytic reactor 16 has been described in accordance with thepreferred embodiment of the present invention as consisting of threeparallel catalyst bed segments 48, 50, 52. However, it will beappreciated that without departing from the essence of the presentinvention the catalytic reactor 16 may equally well consist of any othernumber of catalyst bed segments arranged in parallel relation to eachother, as long as a sufficient number of bed segments are provided,which are capable of remaining in operation while the other ones of thecatalyst bed segments are regenerated by virtue of the passagetherethrough of the regenerating gas. Furthermore, the catalytic reactor16 has been illustrated in accordance with the preferred embodiment ofthe present invention as consisting of a single catalyst bed layer 46.However, it will be appreciated that a catalytic reactor may equallywell also be designed with several, such as, by way of exemplificationand not limitation, 2-5 catalyst bed layers arranged in series, withoutdeparting from the essence of the present invention.

It has been described hereinbefore, without being limited thereto, thatthe condensate from the condenser 40 comprises sulphur trioxide, SO₃. Itis possible that other compounds may be formed, in addition to, or asalternative to, sulphur trioxide, during the regeneration of thecatalytic reactor 16. Examples of such compounds which may be containedin the condensate include ammonium(bi)sulphate and sulphuric acid.

Hereinbefore it has previously been described, with reference to FIGS. 2and 3, how closing devices can be arranged in accordance with thepresent invention so as to be operative for the purpose of isolating acatalyst bed segment from the flow therethrough of the flue gas. It willbe appreciated, however, that other mechanical arrangements couldequally well be utilized without departing from the essence of thepresent invention for purposes of effecting such isolation. One suchalternative arrangement is described and illustrated in U.S. Pat. No.6,340,002, the teachings of which are hereby incorporated herein byvirtue of this reference thereto.

Above it has been described that the catalytic reactor 16 is locateddownstream of the air-preheater 6 and downstream of the electrostaticprecipitator 12. It will be appreciated that other arrangements could beutilized as well. For example, the catalytic reactor could be locatedimmediately downstream of the air-preheater, with a dust removal device,such as an electrostatic precipitator, located downstream of thecatalytic reactor. Furthermore, the catalytic reactor could also belocated immediately downstream of the boiler, with the dust removaldevice and the air preheater being located downstream of the catalyticreactor.

While the present invention has been described herein with reference toa number of preferred embodiments, it will be appreciated by thoseskilled in the art that various changes may be made thereto and/orequivalents may be substituted for various ones of the elements thereofwithout departing from the essence of the present invention. Inaddition, many modifications may be made to the present invention inorder to adapt the present invention for use in a particular situationwithout departing from the essence of the present invention. Therefore,it is intended that the present invention not be limited to theparticular embodiments disclosed herein as the best mode contemplatedfor carrying out the present invention, but that the present inventionshall be considered to include all embodiments of the present inventionthat are deemed to fall within the scope of the appended claims in thisapplication. Moreover, the use of the terms first, second, etc. are notto be considered to denote any order or importance, but rather suchterms first, second, etc., as employed herein, are to be considered asbeing employed simply for the purpose of distinguishing one element fromanother.

Thus, by way of a summary, a catalytic reactor 16 for removing nitrogenoxides, NOx, from a process gas consists of at least two catalyst bedsegments 48, 50, 52, each of which is provided with a closing device 60,62, 64. The catalytic reactor 16 is designed to be operative forpurposes of causing the process gas F to flow through a first bedsegment 48. Moreover, the process gas F is at a first temperature atwhich sulphur trioxide, SO₃, which is entrained in the hot process gas,is at least partially precipitated on to the catalytic material that thefirst catalyst bed segment 48 embodies. Periodically the closing device60 is designed to be operated in order to thereby effect therewith theisolation of the first bed segment 48 from the flow therethrough of theprocess gas F. In addition, a regeneration system 34, 36, 38 is providedthat is designed to be operative for purposes of causing a regeneratinggas to flow through the first bed segment 48. Furthermore, a sulphurtrioxide, SO₃, removal device 20, which is separate from said catalyticreactor 16, is designed to be operative for purposes of effectingtherewith the removal of the sulphur trioxide, SO₃, from saidregenerating gas.

1. A gas cleaning system operative for removing, at least partially,nitrogen oxide from a hot process gas, said gas cleaning systemincluding a catalytic reactor embodying catalytically active material,said catalytic reactor comprising a catalyst bed comprising at least twocatalyst bed segments, which are arranged in parallel relation withrespect to the direction of flow of the hot process gas, each of said atleast two catalyst bed segments being provided with a closing device,such that each of said at least two catalyst bed segments can beindividually isolated from the flow of said hot process gas,characterized in that said catalytic reactor is operative for purposesof causing said hot process gas to flow through at least a firstcatalyst bed segment of said at least two catalyst bed segments, saidhot process gas being at a first temperature at which the sulphurtrioxide entrained in said hot process gas is at least partiallyprecipitated out on to the catalytic material that said first catalystbed segment embodies, and for periodically effecting the operation ofsaid closing device in order to thereby isolate said first catalyst bedsegment from the flow therethrough of said hot process gas, while asecond one of said at least two catalyst bed segments remains operativefor purposes of effecting the removal therewith of the sulphur trioxideand of the nitrogen oxide from said hot process gas, said gas cleaningsystem further includes a regeneration system, which is operative forpurposes of causing a regenerating gas to flow through said firstcatalyst bed segment when said first catalyst bed segment is isolatedfrom the flow of said hot process gas, as well as a sulphur trioxideremoval device, which is separate from said catalytic reactor, and whichis operative for purposes of effecting therewith the removal of thesulphur trioxide from said regenerating gas that has been made to flowthrough said first catalyst bed segment.
 2. The gas cleaning system asclaimed in claim 1, further including a regenerating gas supply device,which is operative for purposes of supplying said regenerating gas at asecond temperature, said second temperature being higher than said firsttemperature.
 3. The gas cleaning system as claimed in claim 1, whereinsaid sulphur trioxide removal device is a sulphur trioxide removaldevice selected from the group of devices consisting of wet scrubbers,dry scrubbers, fabric filters, and electrostatic precipitators.
 4. Thegas cleaning system as claimed in claim 3, wherein said sulphur trioxideremoval device is located at a point downstream of the catalytic reactorwith respect to the direction of flow of said hot process gas, and saidsulphur trioxide removal device is operative for purposes of effectingthe removal therewith of the sulphur species that are entrained in saidhot process gas and in said regenerating gas.
 5. The gas cleaning systemas claimed in claim 1, further including a gas cooler which is operativefor purposes of effecting therewith the cooling of said regenerating gasafter said generating gas has flowed through said first catalyst bedsegment and before said regenerating gas is made to flow through saidsulphur trioxide removal device.
 6. The gas cleaning system as claimedin claim 1, further being operative for purposes of effecting the mixingof at least a portion of said regenerating gas that has flowed throughsaid first catalyst bed segment with an absorption medium that is madeto circulate in said sulphur trioxide removal device for the purpose ofeffecting the removal of the sulphur dioxide from said hot process gas.